Normal body temperature varies from person to person and throughout the day (it is typically highest in the afternoon). Normal body temperature is higher in preschool-aged children and highest at about 18 to 24 months of age. However, despite these variations, most doctors define fever as a temperature of 100.4° F (about 38° C) or higher when measured with a rectal thermometer.

Although parents often worry about how high the temperature is, the height of the fever does not necessarily indicate how serious the cause is. Some minor illnesses cause a high fever, and some serious illnesses cause only a mild fever. Other symptoms (such as difficulty breathing, confusion, and not drinking) indicate the severity of illness much better than the temperature does. However, a temperature over 106° F (about 41° C), although quite rare, can itself be dangerous.

Fever can be useful in helping the body fight infection. Some experts think that reducing fever can prolong some disorders or possibly interfere with the immune system's response to infection. Thus, although a fever is uncomfortable, it does not always require treatment in otherwise healthy children. However, in children with a lung, heart, or brain disorder, fever may cause problems because it increases demands on the body (for example, by increasing the heart rate). So lowering the temperature in such children is important.

Infants with a fever are usually irritable and may not sleep or feed well. Older children lose their interest in play. Usually, the higher a fever gets, the more irritable and disinterested children become. However, sometimes children with a high fever look surprisingly well. Children may have seizures when their temperature rises or falls rapidly (febrile seizures—see Febrile Seizures). Rarely, a fever gets so high that children become listless, drowsy, and unresponsive.

How to Take a Child’s Temperature

A child's temperature can be taken from the rectum, ear, mouth, forehead, or armpit. It can be taken with a glass or digital thermometer. Glass thermometers need to be shaken before use to make sure the temperature they show is below the normal body temperature (98.6° F, or about 37° C). Then they must be left in place for 2 to 3 minutes. Digital thermometers are easier to use and give much quicker readings (and usually give a signal when they are ready). Glass thermometers containing mercury are no longer recommended because they can break and expose people to mercury.

Rectal temperatures
are most accurate. That is, they come closest to the child's true internal body temperature. For a rectal temperature, the bulb of the thermometer should be coated with a lubricant. Then the thermometer is gently inserted about 1/2 to 1 inch (about 1 1/4 to 2 1/2 centimeters) into the rectum while the child is lying face down. The child should be kept from moving.

Ear temperatures
are taken with a digital device that measures infrared radiation from the eardrum. Ear thermometers are unreliable in infants under 3 months old. For an ear temperature, the thermometer probe is placed around the opening of the ear so that a seal is formed, then the start button is pressed. A digital readout provides the temperature.

Oral temperatures
are taken by placing a glass or digital thermometer under the child's tongue. Oral temperatures provide reliable readings but are difficult to take in young children. Young children have difficulty keeping their mouth gently closed around the thermometer, which is necessary for an accurate reading. The age at which oral temperatures can be reliably taken varies from child to child but is typically after age 4.

Forehead (temporal artery) temperatures
are taken with a digital device that measures infrared radiation from an artery in the forehead (the temporal artery). For a forehead temperature, the head of the thermometer is moved lightly across the forehead from hairline to hairline while pressing the scan button. A digital readout provides the temperature. Forehead temperatures are not as accurate as rectal temperatures, particularly in infants under 3 months old.

Armpit temperatures
are taken by placing a glass or digital thermometer in the child's armpit, directly on the skin. Doctors rarely use this method because it is less accurate than others (readings are usually too low and vary greatly). However, if caretakers are uncomfortable taking a rectal temperature and do not have a device to measure ear or forehead temperature, measuring armpit temperature may be better than not measuring temperature at all.

Causes

Fever occurs in response to infection, injury, or inflammation and has many causes. Likely causes of fever depend on whether it has lasted 7 days or less (acute) or more than 7 days (chronic), as well as on the age of the child.

Acute fever

Acute fevers in infants and children are usually caused by an infection. Teething does not typically cause fever over 101° F.

Newborns and young infants are at higher risk of certain serious infections because their immune system is not fully developed. Such infections may be acquired before birth or during birth and include sepsis (a serious bodywide infection), pneumonia, and meningitis.

Children under 3 years old who develop a fever (particularly if their temperature is 102.2° F [39° C] or higher) sometimes have bacteria in their bloodstream (bacteremia). Unlike older children, they sometimes have bacteremia with no symptoms besides fever (called occult bacteremia—see Occult Bacteremia). Vaccines against the bacteria that usually cause occult bacteremia (
Streptococcus pneumoniae and
Haemophilus influenzae type B [HiB]) are now widely used in the United States and Europe. As a result, occult bacteremia is less common. However, pneumococcal strains that are not a part of the current pneumococcal vaccine or other bacteria can sometimes cause it.

Less common causes
of acute fevers include side effects of vaccinations and of certain drugs, bacterial infections of the skin (cellulitis) or joints (septic arthritis), and viral or bacterial infections of the brain (encephalitis), the tissues covering the brain (meningitis), or both. Heatstroke causes a very high body temperature.

Typically, a fever due to vaccination lasts a few hours to a day after the vaccine is given. However, some vaccinations can cause a fever even 1 or 2 weeks after the vaccine is given (as with measles vaccination). Children who have a fever when they are scheduled to receive a vaccine can still receive the vaccine.

Chronic fever

Chronic fever most commonly results from

A prolonged viral illness

Back-to-back viral illnesses, especially in young children

Chronic fever can also be caused by many other infectious and noninfectious disorders. Infectious causes include hepatitis, sinusitis, pneumonia, pockets of pus (abscesses) in the abdomen, infections of the digestive tract caused by bacteria or parasites, bone infections (osteomyelitis), heart infections (endocarditis), and tuberculosis. Noninfectious causes include Kawasaki disease, inflammatory bowel disease, juvenile idiopathic arthritis or other connective tissue disorders, and cancer (such as leukemia and lymphoma). Occasionally, children fake a fever, or caregivers fake a fever in the child they care for. Sometimes the cause is not identified.

Headache, neck stiffness, confusion, or a combination in an older child

When to see a doctor

Children with fever should be evaluated by a doctor right away if they have any warning signs or are less than 2 months old.

Children without warning signs who are between 3 months and 36 months old should be seen by the doctor if the fever is 102.2° F (39° C) or higher, if there is no obvious upper respiratory infection (that is, children are sneezing and have a runny nose and nasal congestion), or if the fever has continued more than 5 days. For children without warning signs who are over 36 months old, the need for and timing of a doctor's evaluation depend on the child's symptoms. Children who have upper respiratory symptoms but otherwise appear well may not need further evaluation. Children over 36 months of age with fever lasting more than 5 days should be seen by the doctor.

What the doctor does

Doctors first ask questions about the child's symptoms and medical history. Doctors then do a physical examination. A description of the child's symptoms and a thorough examination usually enable doctors to identify the fever’s cause (see Examination of the Febrile Child).

Doctors take the child’s temperature. It is measured rectally in infants and young children for accuracy. The breathing rate is noted. If children appear ill, blood pressure is measured. If children have a cough or breathing problems, a sensor is clipped on a finger or an earlobe to measure the oxygen concentration in blood (pulse oximetry).

As doctors examine children, they look for warning signs (such as an ill appearance, lethargy, listlessness, and inconsolability), noting particularly how children respond to being examined—for example, whether children are listless and passive or extremely irritable.

Occasionally, the fever itself can cause children to have some of the warning signs including lethargy, listlessness, and ill appearance. Doctors may give children fever-reducing drugs (such as ibuprofen) and reevaluate them once the fever is reduced. It is reassuring when lethargic children become active and playful once the fever is reduced. On the other hand, it is worrisome when ill-appearing children remain ill-appearing despite a normal temperature.

Some Common Causes and Features of Fever in Children

Cause

Common Features*

Tests

Acute (lasting 7 days or less)

Respiratory infections due to a virus

A runny or congested nose

Usually a sore throat and cough

Sometimes swollen lymph nodes in the neck, without redness and tenderness

A doctor’s examination

Other infections due to a virus

In some infants or children, no symptoms except fever

A doctor's examination

Gastroenteritis

Diarrhea

Often vomiting

Possibly recent contact with infected people or certain animals or consumption of contaminated food or water

A doctor’s examination

Sometimes examination and testing of stool

Ear infection (otitis media)

Pain in one ear (difficult to detect in infants and young children who do not talk)

Sometimes rubbing or pulling at the ear

A doctor’s examination

Throat infections (pharyngitis)

A red, swollen throat

Pain when swallowing

A doctor’s examination

Sometimes a throat culture or rapid strep test (both done on a sample taken from the back of the throat with a swab)

Occult bacteremia

In children under 3 years old

No other symptoms

Blood tests

Pneumonia

Cough and rapid breathing

Often chest pain, shortness of breath, or both

A doctor's examination

Usually a chest x-ray

Skin infections (cellulitis)

A red, painful, slightly swollen area of skin

A doctor’s examination

Urinary tract infection

Pain during urination

Sometimes blood in urine

Sometimes back pain

In infants, vomiting and poor feeding

Urine tests

Encephalitis (a rare infection of the brain)

Infants:
Sometimes bulging of the soft spots (fontanelles) between the skull bones, sluggishness (lethargy) or inconsolability

Fever that recurs in often predictable cycles with periods of wellness in between

Sometimes mouth sores, sore throat, and swollen lymph nodes

Sometimes chest or abdominal pain

Sometimes family members who have had similar symptoms or have been diagnosed with one of the familial periodic fever syndromes

A doctor's examination during episodes of fever

Blood tests during and between fever episodes

Sometimes genetic testing

Pseudo fever of unknown origin

Usually a misinterpretation of normal fluctuations in body temperature or overinterpretation of frequent, minor viral illnesses

Usually no other symptoms of concern

Normal examination findings

A doctor's examination

Thorough and accurate recording of illnesses and temperatures as well as a description of the overall function of the child and family

Occasionally blood tests to rule out other causes and reassure parents

*Features include symptoms and results of the doctor's examination. Features mentioned are typical but not always present. Disorders that cause chronic fever also cause fever during the first 7 to 10 days.

Testing

For
acute fever,
doctors can often make a diagnosis without testing. For example, if children do not appear very ill, the cause is usually a viral infection; a respiratory infection if they have a runny nose, wheezing, or a cough; or gastroenteritis if they have diarrhea and vomiting. In such children, the diagnosis is clear, and testing is not needed. Even if no specific symptoms suggest a diagnosis, the cause is still often a viral infection in children who otherwise do not appear very ill. Doctors try to limit testing to children who may have a more serious disorder. The chance of a serious disorder (and thus the need for tests) depends on the child's age, symptoms, and overall appearance, plus the particular disorders the doctor suspects (see Examination of the Febrile Child).

If
newborns
(28 days old or younger) have a fever, they are hospitalized for testing because their risk of having a serious infection is high. Testing typically includes blood and urine tests, a spinal tap (lumbar puncture—see Tests for Brain, Spinal Cord, and Nerve Disorders : Spinal Tap), and sometimes a chest x-ray.

In
infants between 1 month and 3 months old,
blood tests and urine tests (urinalysis) and cultures are done. The need for hospitalization, a chest x-ray, and a spinal tap depends on results of the examination and blood and urine tests, as well as how ill or well infants appear and whether a follow-up examination can be done. Testing in infants under 3 months old is done to look for occult bacteremia, urinary tract infections, and meningitis. Testing is necessary because the source of fever is difficult to determine in infants and because their immature immune system puts them a high risk of serious infection.

If
children aged 3 to 36 months
look well and can be watched closely, tests are not needed. If symptoms suggest a specific infection, doctors do the appropriate tests. If children have no symptoms suggesting a specific disorder but look ill or have a temperature of 102.2° F (39° C) or higher, blood and urine tests are usually done. The need for hospitalization depends on how well or ill children look and whether a follow-up examination can be done.

In
children over 36 months old,
tests are typically not done unless children have specific symptoms suggesting a serious disorder.

For
chronic fever,
tests are often done. If doctors suspect a particular disorder, tests for that disorder are done. If the cause is unclear, screening tests are done. Screening tests include a complete blood cell count, urinalysis and culture, and blood tests to check for inflammation. Tests for inflammation include the erythrocyte sedimentation rate (ESR) and measurement of C-reactive protein (CRP) levels. Other tests doctors sometimes do when there is no clear cause include stool tests, tuberculosis tests, chest x-rays, and computed tomography (CT) of the sinuses.

Rarely, fevers persist, and doctors cannot identify the cause even after extensive testing. This type of fever is called fever of unknown origin (see Biology of Infectious Disease:Testing). Children with a fever of unknown origin are much less likely to have a serious disorder than are adults.

Treatment

If the fever results from a disorder, that disorder is treated. Other treatment focuses on making children feel better.

General measures

Ways to help children with a fever feel better without using drugs include

Placing children in a warm bath (only slightly cooler than the temperature of the child)

Because shivering may actually raise the child’s temperature, methods that may cause shivering, such as undressing and cool baths, should be used only for dangerously high temperatures of 106° F (about 41° C) and above.

Rubbing the child down with alcohol or witch hazel must not be done because alcohol can be absorbed through the skin and cause harm. There are many other unhelpful folk remedies, ranging from the harmless (for example, putting onions or potatoes in the child's socks) to the uncomfortable (for example, coining or cupping).

Drugs to lower fever

Fever in an otherwise healthy child does not necessarily require treatment. However, drugs called antipyretic drugs may make children feel better by lowering the temperature. These drugs do not have any effect on an infection or other disorder causing the fever. However, if children have a heart, lung, brain, or nerve disorder or a history of seizures triggered by fever, using these drugs is important because they reduce the extra stress put on the body by fever.

Typically, the following drugs are used:

Acetaminophen, given by mouth or by suppository

Ibuprofen, given by mouth

Acetaminophen tends to be preferred. However, some doctors are concerned that acetaminophen use has contributed to the recent increase in asthma in children and thus do not recommend its use in children with asthma or who have a family history of asthma. Ibuprofen, if used for a long time, can irritate the stomach’s lining. These drugs are available over the counter without a prescription. The recommended dosage is listed on the package or may be specified by the doctor. It is important to give the correct dose at the correct interval. The drugs do not work if too little drug is given or it is not given often enough. And although these drugs are relatively safe, giving too much of the drug or giving it too often can cause an overdose.

Rarely, acetaminophen or ibuprofen is given to prevent a fever, as when infants have been vaccinated.

Aspirin is no longer used for lowering fever in children because it can interact with certain viral infections (such as influenza or chickenpox) and cause a serious disorder called Reye syndrome (see Reye Syndrome).

Key Points

Usually, fever is caused by a viral infection.

The likely causes of fever and need for testing depend on the age of the child.

Infants under 2 months of age with a temperature of 100.4° F or higher need to be evaluated by a doctor.

Children aged 3 to 36 months with fever who have no symptoms suggesting a specific disorder but look ill or have a temperature of 102.2° F (39° C) or higher need to be evaluated by a doctor.

Teething does not cause significant fever.

Drugs that lower fever may make children feel better but do not affect the disorder causing the fever.

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